What does RMS voltage signify?

[david90]

Is it like an average voltage or something?

 

[Regnum]

Re: what is RMS voltage?

The RMS value of an AC is the DC voltage that produces the same average power as the AC, when applied on a resistor.

 

[tutx]

what is RMS voltage?

V(RMS) is Root-Mean-Square Voltage.

V(rms) = (V*V)**(1/2)

For a sinusoidal voltage:

V(rms) = (Vo*Vo*Cos(wt)*Cos(wt))**(1/2) = Vo/(2**(1/2))

that's all !

 

[pixel]

Re: what is RMS voltage?

Average voltage tells you that your voltage fulctuates around some average value.
RMS voltage shows you how much is that fluctuation.
Square of RMS could be understood as the average power on a resistor of 1 Ohm.

 

[Hughes]

Re: what is RMS voltage?

Root Mean Square voltage

 

[july]

Re: what is RMS voltage?

Hi,

Can anyone tell me how to calculate the RMS values using MCU?

 

[Hughes]

what is RMS voltage?

VRMS=sqrt(sum(V*V)/N), n=1~N

 

[awan]

Re: what is RMS voltage?

see D.J IRVIN BOOK ON THE CIRCUIT ANALYSIS CHAP 11

 

[golfbumb]

Re: what is RMS voltage?

The RMS voltage of a signal is computed by squaring the instantaneous voltage, integrating over the desired time, and taking the square root. (Refer to "Hughes" integral formula in his previous post).

The RMS computation is ultimately used to give a measure of the average continous power carrying capability of a signal. The instananeous voltage values are squared (the V^2 term) which is then summed up (the integration) before converting back to voltage by the square root operation. Once the RMS voltage value is known, then you can make accurate estimates on true power delivery over time, independent of the signal's polarity.

 

[Kevin Weddle]

Re: what is RMS voltage?

If you take the RMS and square it you get the average of the sin x squared when dealing with the sign wave. So a sign wave with an amplitude of 3 has an RMS of 2.121. Square that and you get 4.49. This is the average of the 3sin x quantity squared. You have to know that the sin x squared is different from the sin x.

 

[djalli]

what is RMS voltage?

Why not rule of thumb:
(Vpeak-to-peak)*(0.707) = Vrms

(Vrms)/(0.707) = Vpeak-to-peak

?

 

[nicleo]

Re: what is RMS voltage?

Why not rule of thumb:
(Vpeak-to-peak)*(0.707) = Vrms

(Vrms)/(0.707) = Vpeak-to-peak

?

This formula only applied to PURE sinewave signal. Nowsaday many electrical appliances, e.g. adjustable speed drives, tend to introduce harmonics into electrical system. Consequently, the signal might not be PURE sinewave anymore.


As incandescent light bulb (100W) is mainly built from resistive material, the current waveform can be considered as sinewave (only fundamental harmonic appears in current harmonic spectrum). The power factor of this device is UNITY.

Fluorescent lamp and air-cond introduce little harmonic into the electrical system. The power factor is not unity.

Computer and TV (mainly built from semiconductor devices - nonlinear) introduce a lot harmonic into the electrical system.


That's why we have TRUE RMS meter now. This meter takes into consideration the fundamental and harmonic content of a given signal.

 

[djalli]

what is RMS voltage?

nicleo you are absolutely correct. But here is the thing:

Once you know spacing of each component of harmonics you can still apply the same formula I said above for each harmonics. It becomes Fourier series.

World can Also be composed with sinusoids.

 

[nicleo]

what is RMS voltage?

djalli, you're absolutely correct too.

Can anyone tell me how to calculate the RMS values using MCU?

But to reply july's problem, in my opinion, it's a little bit easier to perform method proposed by Hughes as quoted below:

VRMS=sqrt(sum(V*V)/N), n=1~N

If I'm not mistaken, we need to perform FFT in microcontroller to obtain all harmonics, which I think the FFT method will consume more computational power.

 

[july]

Re: what is RMS voltage?

If I'm not mistaken, we need to perform FFT in microcontroller to obtain all harmonics, which I think the FFT method will consume more computational power

Do you know some project which concern?

Thanks

 

[nill]

Re: what is RMS voltage?

The RMS volatage term is used for alternating volatage. It means this: The roots mean square (rms) of an alternating voltage is the value of the direct p.d. which when applied across the same resistor, produces heat at the same rate as the mean rate of heat produced by the alternating volatge.

But literally RMS means take the SUM, get the MEAN and then take the ROOT; in that order, and you'll get the RMS value.

 

[wangsx]

what is RMS voltage?

Hi
here is the relation between RMS and The Standard deviation Q as below:
RMS=sqrt(sum(V^2))/N), n=1~N
Q=sqrt(sum((V-u)^2/N), n=1~N
when u=0, RMS=Q.

 

[nicleo]

Re: what is RMS voltage?

If I'm not mistaken, we need to perform FFT in microcontroller to obtain all harmonics, which I think the FFT method will consume more computational power

Do you know some project which concern?

Thanks

I'm sorry that I don't have any project that performs RMS calculation based on FFT using MCU. The data for those plots in my previous post were obtained from Fluke Power Analyser. Normally, we acquire/sample the voltage/current and then send the data to PC to process (e.g. using Matlab or writing our own program to calculate). Well, if you wish to implement FFT method to find harmonics and therefore the RMS value of voltage or current, probably you would like to consider using FFTW (Fastest Fourier Transform in the West)( http://www.fftw.org/ ). Some people claim FFTW is more efficient than conventional FFT. By the way, do you mind to share some background on your application?

This one is another reference (not really a project)
Influence of Digital Signal Processing on Precision of Power Quality Parameters Measurement
http://www.measurement.sk/2003/S1/Szolik.pdf

Development of a Measurement System for Power Quantities in Electrical Energy Distribution Systems
**broken link removed**

 

[cetc1525]

what is RMS voltage?

it is most used for test the noise of power supply.

 

[july]

Re: what is RMS voltage?

Thanks, nicleo. Actually, this is a my friend's question. He found that DSP was used in some electronic watt-hour meter brand. I also want to know how the voltage and current are calculated.